Orthoquinodimethanes

Oligomers Oppolzer s camphor sultam (see also camphor derivatives) Organometallics Organosilicon Organosulfur Organozirconocenes Orthoquinodimethanes Oxabutadienes Oxanorbornadiene(s) Oxazinolactams Oxazinones Oxazoles... 

Further examples of the use of the hDA reaction in dihydropyran synthesis include the formation of the fused pyrans 18 from vinylallenes 17 and aldehydes (Scheme 8) <00TL6781> and a trans-fused dihydropyran containing a phosphonate group 19 . A total synthesis of the 11-oxa steroid system is based on an intramolecular Diels-Alder reaction involving an orthoquinodimethane derived from a benzocyclobutene (Scheme 9) <00TL1767>. 

It is possible for organosilyl compounds with a leaving group at the fi position to undergo E2-type reactions induced by cesium fluoride. In deprotonation-ehmina-tion, a strong base such as amide or alkoxide is required the conjugated acid produced by deprotonation (e.g. amine or alcohol) may react with the elimination product. In the case of cesium fluoride-induced desilylation-elimination, the formed conjugated acid is silyl fluoride which is unreactive. The production of unstable products such as cyclopropene (Scheme 2.13), cyclohexene-3-yne (Scheme 2.14), or orthoquinodimethane has been reported [24—29]. 

Although the mechanismis not a formal Sn27 reaction, the base-catalyzed 1,4-eliminations of 31 proceed smoothly and generate the difluorinated orthoquinodimethane-type intermediates ) [18]. Likewise, 29 [19] and30 [20] have been generated. All of these intermediates are very reactive toward nucleophilic alkylation at the difluoromethylene site. The overall chemical transformations provide a variety of highly functionalized fluoroheterocycles (Scheme 2.16). 

Development of synthetic routes based on generation and intramolecular cycloaddition of orthoquinodimethanes. 

In the previously discussed routes the requisite orthoquinodimethanes were... 

A novel procedure for generation of orthoquinodimethanes, developed and exploited by Vollhardt, is based on the co-oligomerization of a-a>-diacetylenes and monoacetylenes in the presence of cyclopentadienylcobalt dicarbonyl catalyst. " The reaction is carried out by addition of the diacetylene and catalyst to excess refluxing monoacetylene. When n = 2, the benzocyclobutene co-oligomerization product formed in situ is in thermal equilibrium with the corresponding o-quinodimethane, which can undergo Diels-Alder cyclization with an appropriately positioned dienophile. 

The use of bicyclo(2,2,l)-heptane derivatives in natural product synthesis has been explored extensively by Grieco and co-workers. They have devised a synthesis of ( )-estrone based on orthoquinodimethane methodology in which e ring D component is derived stereospecihcally from the bicyclo-(2,2,l)heptane species 134 (Scheme 18). The latter and related bicycloheptanes are readily available from norbomadiene by a sequence which features e Prins reaction, devised independently by groups led by Corey and Sutherland in conjunction with their studies on prostaglandin synthesis. 

Tsuji and his group have developed a palladium catalyzed cyclization reaction that has led to a simple synthesis of 2-carbomethoxy-3-vinylcyclopentanone 142, which they have utilized in syntheses of cyclopentanoid natural products. In the steroid field, 142 served as a ring D component in an orthoquinodimethane-based route to ( )-18-hydroxyestrone. The (-I-)-diastereomer is a component of pregnancy urine. ( )-Estradiol-3-methyl ether was also obtained (Scheme 19). ... 

Saegusa and co-workers have shown that orthoquinodimethanes can be generated by fluoride ion desilylation of ortho-(a-trimethylsilylalkyl)benzyltrimethyl ammonium salts at temperatures in the 20 to 80° range,strikingly lower than the ca. 180-220° range required to generate orthoquinodimethanes from benzocy-clobutenes or benzodihydrothiophene dioxides. 

Magnus et al. have devised a route to ( )-lla-hydroxyestrone 3-methyl ether which, like the Saegusa synthesis (Scheme 20), makes use of fluoride mediated elimination of the trimethylsilyl group, but which is coupled with alternative methodology for in situ orthoquinodimethane generation (Scheme 21). ... 

The preparation of 2,3-dimethylene heteroaromatics (DMHAs), as analogues of orthoquinodimethanes, has attracted a good deal of attention in recent years. Such systems are often quite labile, and one of the most commonly employed synthetic strategies to them has been to build the required heteroaromatic ring fused to a sulfolene, as a direct precursor to the DMHA [79]. Some interesting work has been done on Diels-Alder reactions from DMHAs derived from fused sulfolenes, and these are covered in more detail in Section 6.5. 

The complex [Fe(CO)3(ri" -13-cyclohexadiene)], useful in organic synthesis, is obtained by complexation-isomerization of the 1,4-isomer by reaction with [Fe(CO)5]. There are also other robust 18-electron complexes of unsaturated hydrocarbons as L2 ligands that are related to dienes but are instable in the free state cyclobutadiene, orthoxylylene (orthoquinodimethane) and trimethylenemethane (below). 

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